(a) Field of the Invention
The present invention relates to a liquid crystal display and driving method thereof, and more specifically, to a liquid crystal display and driving thereof to highly increase the speed of bend alignment at initial operation such as right after power application in a liquid crystal display with an OCB mode.
(b) Description of the Related Art
In general, since a liquid crystal display (LCD) is much thinner, much light-weighted and less consumed in power than a cathode ray tube prevailing in an image display device at present, it is already wide-used as an image display device of portable information devices such as a mobile phone and a notebook computer, and it is so small in radiation of an electromagnetic wave that it is expected to prevail in an image display device for table instead of the cathode ray tube.
Such an LCD has a disadvantage that a viewing angle feature of changing brightness and color is large depending on a direction where a screen is seen. Several methods to overcome this advantage have been proposed.
For example, a method, which increases an orthogonal brightness more than 30 percent by attaching a prism plate to surface of light transmitting plate to increase traveling straight of an incident light in order to improve the viewing angle of an LCD, has been put to practical use, and another method, which attaches a negative optical compensating plate to surface of light transmitting plate to increase the viewing angle, is being used.
In addition, owing to an In Plane Switching mode, a wide viewing angle almost equal to a CRT level is accomplished because a viewing angle in all direction is 160°, but it is so low in an aperture rate that it is needed to improve this.
Besides the above methods, a variety of trials to improve a viewing angle are being made by driving methods such as OCB (Optical Compensated Birefringency) method, PDLC (Polymer Dispersed Liquid Crystal) method and DHF (Deformed Helix Ferroelectric) method by a TFT.
Especially, since the OCB mode has advantages that response speed of liquid crystal is high and its viewing angle is wide, it is vigorously in the process of research and development.
An operation of the above OCB mode will be described in brevity with reference to FIG. 1.
FIG. 1 is a diagram to illustrate an operation of a normal OCB mode, and FIG. 2 is a diagram to illustrate ON/OFF cycle of the OCB mode.
Referring to FIG. 1, an initial alignment state of the liquid crystal that poses between an upper plate electrode and a lower plate electrode is a Homogeneous state (hereinafter referred to as “H”). When a specific voltages is applied to the upper/lower plate electrode, its state is changed through a Transient splay (hereinafter referred to as “T”) and an Asymmetric splay (hereinafter referred to as “A”) to a Bend state (hereinafter referred to as “B”), and then it works as an OCB mode.
As shown in FIG. 1, generally, an OCB liquid crystal cell is made to have pretilt angle of about 3°˜20° in the vicinity of alignment film, the thickness of the liquid crystal cell is made to be 4˜10 μm and the alignment film is made to be rubbed in the same direction. Since the arrangement of the liquid crystal molecules in the center of the liquid crystal layer is symmetry for left and right side, a tilt angle becomes 0° below a specific voltages, and 90° over a specific voltages. Therefore, by applying a large voltages to the liquid crystal, it is made to be 90° for tilt angle of liquid crystal molecules in the center of liquid crystal layer, and by changing the voltages applied thereto, a tilt of vicinity of the alignment film and liquid crystal molecules in the middle layer except for the liquid crystal molecules layer in the center of the liquid crystal layer is made to be changed, and thus polarization of the light transmitting through the liquid crystal layer is modulated.
Although it typically takes several seconds for tilt angle to arrange from 0° to 90°, the OCB liquid crystal cell has a feature that response time is very short such an extent of 10 ms because of voltages change thereafter with no back-flow and a bend transformation with an high elastic coefficient.
As shown in FIG. 2a, in the ON state of normal OCB mode, its change is rapid from T to A and is relatively rapid from T to B, but is slow from A to B, as also shown in FIG. 2b, in the OFF state of normal OCB mode, its change is slow from B to H, but is rapid from T to H or from A to H.
As described above, there is a problem that it takes a certain time to obtain a bend alignment for an OCB mode. In particular, there is a problem that it is possible to use an LCD, by applying a large voltages thereto for a short time after turning on a power switch of a PC monitor or a TV to have to induce bend alignment transition for the whole panel of the LCD panel.